During the manufacture of semiconductor devices such as dynamic random access memories, static random access memories, logic devices, and microprocessors, several structures are commonly formed. For example, conductive interconnects such as word lines, and conductive plugs such as digit line contact plugs, are commonly used.
A common engineering goal during the design of semiconductor devices is to manufacture as many features in a given area as possible. An obvious method to aid in accomplishing this goal is to make feature sizes smaller. One problem resulting from smaller feature sizes is that decreasing the width of a conductive line or conductive plug decreases the cross-sectional area of the line or plug, which in turn increases the resistance of the feature. Features which were originally manufactured from only conductively-doped polysilicon, which has a relatively high resistance, required the formation of a lower resistance material to decrease the overall resistance of the feature, for example an overlying layer of silicide such as tungsten silicide.
As device feature sizes further decrease it becomes desirable to form the entire feature from a highly conductive material such as a metal, for example cobalt. Deposition techniques for metals include various methods. During one sputter process, a target manufactured from the deposition material may be bombarded with ions to erode the material from the target and redeposit it onto a semiconductor wafer substrate assembly. In another sputter process, the target material may be reacted with gas phase species to form compound films. During chemical vapor deposition (CVD), gasses are mixed within a chamber and chemically combine on the wafer substrate surface to form a film. Both sputtering and CVD result in the continued increase in thickness of the material layer being deposited during the process. As long as the process continues the thickness of the layer increases. Both CVD and sputtering can be used to deposit oxides, nitrides, and metals.
Thickness uniformity of a layer formed by either CVD or sputtering depends on a variety of factors. For CVD, the gasses to be combined must be dispersed uniformly above the surface receiving the layer to be deposited, otherwise the layer may form to a greater thickness in the areas of higher gas concentrations. The uniformity of a sputtered layer is highly dependent on the topography of the surface receiving the deposited layer. Since the deposition process is line-of-sight, it is difficult to deposit films in the bottom of a deep feature having a high aspect, or depth to width, ratio.
Another method used to form a layer is atomic layer deposition (ALD). With ALD, a precursor is introduced within a chamber to bond with free binding sites on the surface of the wafer substrate assembly to form a layer which is a single atom or molecule thick. Once all the binding locations are full, chemical deposition stops regardless of how much vapor remains in the chamber. This precursor gas is purged from the chamber and a reaction gas is introduced which reacts with the adsorbed precursor until all surface sites are reacted, saturating the surface and regenerating a surface which will again react with the precursor. ALD has an advantage over CVD and sputtering in that it forms a highly conformal layer over severe topography. ALD is typically used to form dielectrics such as oxides and nitrides, for example metal oxides and metal nitrides. The deposition of some pure metals with ALD has also been proven to be possible. However, the formation of pure metal cobalt on bare silicon, for example by exposing bare silicon to a cobalt precursor, has not proven possible. Cobalt can be formed on a bare silicon wafer but only after the wafer has been exposed to air for at least 15 minutes. During this time the wafer surface becomes fully hydrated, or the first monolayer of oxide is formed. Thus ALD cobalt can typically only be grown on this thin oxide/oxygen layer. Oxide/oxygen formation has deleterious effects, for example increasing the resistance between the silicon wafer and a cobalt/cobalt silicide contact.
A method which enables atomic layer deposition of a metal such as cobalt on atomically clean bare silicon would be desirable.